Refer to FIG. 1 a diagram schematically showing a conventional active clamp circuit. In the conventional active clamp circuit, a pulse modulation unit 5 controls a primary switch unit 11 and a secondary switch unit 12 to alternately turn on and thus controls the excitation current of a transformer 2 to modulate the output power. The output power is converted into DC power via a rectification unit, which comprises two switch elements SW3 and SW4. An output feedback unit 4 generates a feedback signal and transmits the feedback signal to the pulse modulation unit 5, which modulates output responsively with respect to the transient variation of the load. Refer to FIG. 2 for the waveforms thereof. When the transient current variation is so great that the current is over the coil-saturation critical line 103, the transformer is saturated, and a surge current 102 appears. The IC chips used by the current pulse modulation unit 5 all have the over-current protection function. However, the over-current protection function only limits the sum of the excitation current and the load current. Thus, when the excitation current is very high, the protection function may be unable to stop the increase of the excitation current and prevent the transformer from saturation because the load current is very low and the sum of the excitation current and the load current does not exceed the limit. The protection function will turn off the primary switch 11 to diminish the surge current as soon as the transformer is saturated and the surge current appears. However, the surge current, which appears momentarily before the primary switch unit 11 is turned off, will generate inductions back and forth between two sides of the transformer and will generate a very high surge voltage 101 on the primary switch unit 11. The surge voltage 101 may seriously damage the power supply, which is a problem the conventional technology has to face. A conventional technology “Optimal Resetting of the Transformer's Core in Single-Ended Forward Converters” disclosed in a U.S. Pat. No. 4,441,146 utilizes two switches to perform simple turn-on operations alternately. However, the conventional technology lacks the mechanism to control the turn-on timing of the two switches. Thus, in the transient state, the transformer is likely to be saturated, which may result in inferior transient-state response, transient-state noise, and over voltage. Consequently, the quality of the output power may be degraded, and the elements of the circuit may be damaged. Another conventional technology “Finite Voltage-Clamp Forward-Conversion Power Supply” disclosed in a R.O.C patent publication No. 312869 utilizes a clamp inductor and diodes to store redundant energy into a capacitor. When the inductance of the clamp inductor is equal to the inductance of the primary-side coil of the transformer, the charge and discharge capabilities of the capacitor are identical, and the saturation of the transformer can be inhibited. However, the abovementioned extra elements raise power consumption and decrease efficiency. Besides, the inductance of the clamp inductor must be identical to the inductance of the primary-side coil of the transformer. If the inductances are different, the mismatch between the clamp inductor and the primary-side coil occurs, and the energy flow will be unbalanced. In fact, those two inductors are hard to match. Therefore, the control method of the active clamp circuit still needs improving to simultaneously have the advantages of simplicity, non-saturation and low power consumption.